Synthesis and characterization of modified Schiff base silatranes (MSBS) via ‘Click Silylation’

Journal of Molecular Structure 1079 (2015) 173–181

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Synthesis and characterization of modified Schiff base silatranes (MSBS) via ‘Click Silylation’ Gurjaspreet Singh a,⇑, Aanchal Arora a, Satinderpal Singh Mangat a, Jandeep Singh a, Sunita Chaudhary a, Navneet Kaur b, Duane Choquesillo-Lazarte c,⇑ a b c

Department of Chemistry and Centre of Advanced Studies, Panjab University, Chandigarh 160014, India Centre for Nano-Science and Nano-Technology, Panjab University, Chandigarh 160014, India Laboratorio de Estudios Cristalográficos, IACT-CSIC, Avda. de las Palmeras 4, 18100 Armilla, Granada, Spain

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Schiff bases (1a–1d) were used for the

Modified Schiff base silatranes (4a–4d) derived from novel terminal alkynes (2a–2d) were synthesized and fully characterized by IR, NMR (1H, 13C), mass spectroscopy and elemental analysis. Molecular structures of compounds 2b and 4b were authenticated using X-ray crystallographic studies. Highly potential ‘Click Silylation’ route is followed for the synthesis.

synthesis of substituted terminal alkynes (2a–2d).  1,2,3-Triazole capped triethoxysilanes (3a–3d) were obtained by ‘Click Silylation’.  All the newly synthesized compounds were well characterized.  Silatranes with Schiff base as an exocyclic moiety.

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 14 August 2014 Received in revised form 12 September 2014 Accepted 13 September 2014 Available online 20 September 2014

Schiff bases (1a–1d) were modified into terminal alkynes (2a–2d) which on Click Silylation with 3-azidopropyltriethoxysilane (AzPTES) yielded 1,2,3-triazole capped triethoxysilanes (3a–3d). These triethoxysilanes on transesterification with triethanolamine afforded corresponding modified Schiff base silatranes (MSBS) (4a–4d) in high yield and purity. All the synthesized compounds were well characterized by IR, NMR (1H, 13C), mass spectroscopy, elemental analysis and complete structure elucidation by Xray diffraction studies for 2b and 4b. Starting alkynes and final silatranes are further compared by their absorption spectra and TGA analysis. Synthesized MSBS are the first compounds of their kind which being hydrolytically stable can be put to further use in the field of medical and material research. Ó 2014 Elsevier B.V. All rights reserved.

Keywords: Schiff base Terminal alkyne Triethoxysilane Click Silylation Silatrane

Introduction ⇑ Corresponding authors. Tel.: +91 0172 2534428, +91 9814 302099 (G. Singh). E-mail addresses: [email protected] (D. Choquesillo-Lazarte).

(G.

http://dx.doi.org/10.1016/j.molstruc.2014.09.042 0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

Singh),

[email protected]

Initiated by Sharpless and co-workers, ‘Click Chemistry’ has emerged as a powerful strategy for the molecular stitching of

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G. Singh et al. / Journal of Molecular Structure 1079 (2015) 173–181

smaller fragments into complex molecules [1]. Click route has now established its footmarks owing to its remarkable efficiency, specificity, bioorthogonality and high yields, thus meeting the demands of modern day chemistry [2,3]. Pioneering work by Cattoen and co-workers prompted us to synthesize 1,2,3-triazole linked oragnotriethoxysilanes (OTES) via ‘Click Silylation’ [4]. This copper catalyzed azide-alkyne cycloaddition (CuAAC) reaction is gaining wide popularity for the synthesis of OTES [5]. The synthesis of diverse alkynyl frameworks and their linkage to azide functionality is one of the most powerful synthetic routes to triazole derivatives and shows high chemoselectivity because of incredible functional group tolerance [1,6–11]. 1,2,3-Triazole moiety has emerged as an important 5-membered N-heterocycle that has found wide range of applications including biochemicals, agrochemicals, drugs, photostabilizers and corrosion inhibitors [12–16]. Recently, Singh et al. have reported the synthesis of OTES and silatranes substituted with various terminal polyfunctionalized molecular entities via 1,2,3-triazole linkage and explored their role as chemosensing agents [5,17–19]. OTES being hydrolytically unstable and sensitive to agglomeration and polymerization are preferably modified into more stable silatrane analogues. Silatranes, hetero-tricyclic compounds with transannular N ? Si bond have gained continuous attention due to their high stability, unique structure, architectural beauty and wide applications in agriculture and biology [20–22]. These are cage-like silico-organic cyclic ethers of tris(2-oxyalkyl)amine with general formula R–Si(OCH2CH2)3N and distorted trigonal bipyramidal geometry at the silicon atom [23]. Silatranyl group has an important stereoelectronic influence in shaping the reactivity of the exocyclic functional group apical to the transannular bond [24,25]. Derivatization of silatranes with different groups in exocyclic position is an important trend as the resulting compounds retain the activities of silatranes as well as substituted groups [26,27]. Schiff bases [28–31] are amongst such groups that can be linked with silatranes [32–36] owing to their potential chemical and biological activity [37–39]. Several Schiff base substituted silatranes have been reported in literature [40,41] but it is for the first time in organosilicon chemistry that the Schiff bases are linked to silatranes via 1,2,3-triazole linker. Schiff base bound triazole derivatives have been synthesized by ‘Click Silylation’ of Schiff base modified terminal alkynes with AzPTES. Blending the Schiff base moiety with biologically and materially important triazole and silatranyl moieties into a single chemical framework may lead to further exploitation of these compounds in various research fields. Experimental section

CHO Ar

N

Ethanol

NH2

4 h reflux

OH

Ar

+

(1a-1d)

OH

K2CO3 o Br DMF, 25 C 16 h

O Ar

N

+ -N O

Si N+

O

O

N

(2a-2d) CuBr(PPh3)3 THF, Et3N 60 oC Ar

O N

Si N

O

(3a-3d)

HO HO

N

O

O

N

Toluene KOH 5 h reflux

OH N

O Ar

O

N

Si N

O

(4a- 4d)

N

O

N

N

Ar = C6H5 , 2-C5H4N Scheme 1. Synthesis of modified Schiff base silatranes (4a–4d).

(AL 300 MHz) spectrometer using CDCl3 as internal reference and chemical shifts were reported relative to tetramethylsilane. HRMS data of all synthesized silatranes 4a–4d was recorded on Waters QQ–TOF micro Mass Spectrometer. Melting points were uncorrected and measured in a Mel Temp II device using sealed capillaries. TGA analysis was run on a SDT Q 600V 20.9 Build 20 TGA Instrument. Alumina pans were used for sample loading and sample ramped at 10 °C/min to 1000 °C in dry air at 60 ml/min.

Materials and methods X-ray structure determination All the syntheses were carried out under dry nitrogen atmosphere using vacuum glass line. The organic solvents were dried according to standard procedures [42]. Aniline (Qualigens, >99%) and salicylaldehyde (CDH) were vacuum distilled prior to use. Propargyl bromide (80% in toluene) (Aldrich), p-hydroxybenzaldehyde (SDFCL), potassium carbonate (Thomas), sodium sulfate (Finar), Bromotris(triphenylphosphine)copper(I) [CuBr(PPh3)3] (Aldrich) were used as received. 3-azidopropyltriethoxysilane (AzPTES) and Schiff bases 1a–1d were synthesized by known procedure from literature [43,44]. Infrared spectrum was obtained neat on a Thermo Scientific Fischer spectrometer. CHN analysis was obtained on Perkin Elmer Model 2400 CHNS elemental analyzer and Thermo Scientific Flash 2000 organic elemental analyzer. Mass spectral measurements (ESI source with capillary voltage, 2500V) were carried out on a VG Analytical (70-S) spectrometer. Multinuclear NMR (1H, 13C) spectra were recorded on a Bruker advance II 400 and on a Jeol

Measured crystals were prepared under inert conditions immersed in perfluoropolyether as protecting oil for manipulation and then they were mounted on a MiTeGen MicromountsTM and this sample was used for data collection. Data were collected with a Bruker D8 Venture diffractometer. Data were processed with APEX2 [45] and corrected for absorption using SADABS [46]. The structures were solved by direct methods [47], which revealed the position of all non-hydrogen atoms. These atoms were refined on F2 by a full-matrix least-squares procedure using anisotropic displacement parameters [47]. All hydrogen atoms were located in difference Fourier maps and included as fixed contributions riding on attached atoms with isotropic thermal displacement parameters 1.2 times those of the respective atom. In compound 4b, the (4-methoxybenzylidene)amine moiety exhibits positional disorder over two positions, with site occupancies of 0.693(10) and 0.307(10). Drawings were produced with Olex2 [48].

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G. Singh et al. / Journal of Molecular Structure 1079 (2015) 173–181 Table 1 Synthesized terminal alkynes (2a–2d), triethoxysilanes (3a–3d) and modified Schiff base silatranes (4a–4d). Terminal alkynes (2a–2d)

Triethoxysilanes (3a–3d)

Silatranes (4a–4d)

Yield of 4a–4d (%) 93

O O

O

Si

O Si

N N O

2a

N

O

N

O

O

O

N

N

N

N

N

N

4a

3a 94

O O

N

O

O

Si

N

Si O

N

2b

N

O N

3b

N

O

4b

O

N

O

N

N

N

93

O O

O

N

Si

O Si

N N O

N

2c

N

O

O

N

O

O

N N

N

N

N

N

N

4c

3c 92

O

N

O

N

N

O

N

O

2d

N

O

3d

N

O

Si

N

Si O

N

N

O

4d

N

O

N

N

N

Synthesis General procedure for synthesis of silanes Schiff base alkyne (1 equiv) was added to 1:1 THF/Et3N solvent mixture taken in a 2-neck round bottomed flask and the mixture was stirred for 15 min at room temperature. Slow addition of AzPTES (1 equiv/alkyne function) was done followed by catalyst [CuBr(PPh3)3] (0.01 mmol/alkyne function) loading. The mixture was stirred at 60 °C for 5 h. The reaction was then brought to room temperature. The solvents were evaporated under reduced pressure followed by the addition of hexane. The reaction mixture was then filtered and concentration of filterate under reduced pressure afforded the viscous brown silane in good yield. Synthesis of 2-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4yl)methoxy)benzylidene) benzenamine (3a). The quantities used were as: 2a (1.0 g, 4.26 mmol), AzPTES (1.05 g, 4.26 mmol). Yield: 1.89 g, 92%. Anal. Calcd. for C25H34N4O4Si: C, 62.21; H, 7.10; N, 11.61. Found: C, 62.18; H, 7.12; N, 11.58. IR (Neat, cm1): 753, 1072 (SiAO), 954 (CAC), 1161, 1241 (OACH2), 1285 (CH2AN), 1454 (CH3AC), 1618 (C@N), 2972 (C@CAH). 1H NMR (300 MHz, CDCl3, 25 °C): d = 0.47 (m, 2H, ASiCH2A), 1.12 (t, 9H, AOCH2CH3, J = 7.1 Hz), 1.93 (m, 2H, ACCH2CA), 3.70 (q, 6H, AOCH2CH3, J = 7.0 Hz), 4.27 (t, 2H, AN3CH2CH2A, J = 7.2 Hz), 5.24 (s, 2H, AOCH2A), 6.95–7.02 (m, 3H, H2AH4), 7.05–7.33 (m, 5H, H8AH12), 7.48 (s, 1H, TzAH), 8.09 (d, 1H, H5, J = 9.0 Hz), 8.78 (s, 1H, ACH@N). 13C NMR (75 MHz, CDCl3, 25 °C): d = 7.6 (SiCH2), 18.4 (CH3), 24.3 (CCH2C), 52.7 (N3CH2CH2), 58.7 (OCH2CH3), 62.9 (OCH2), 113.1 (C2), 113.4 (C6), 121.8 (C8, C12), 123.2 (C4), 126.0 (C10), 128.1 (C9, C11), 129.4 (C5), 133.0 (C3), 121.3, 136.2 (TzAC), 153.1 (C7), 156.6 (CH@N), 158.6 (C1).

Synthesis of 4-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4yl)methoxy)benzylidene) benzenamine (3b). The quantities used were as: 2b (1.0 g, 4.26 mmol), AzPTES (1.05 g, 4.26 mmol). Yield: 1.96 g, 95%. Anal. Calcd. for C25H34N4O4Si: C, 62.21; H, 7.10; N, 11.61. Found: C, 62.13; H, 7.04; N, 11.58. IR (Neat, cm1): 760, 1072 (SiAO), 954 (CAC), 1164, 1244 (OACH2), 1303 (CH2AN), 1491 (CH3AC), 1622 (C@N), 2972 (C@CAH). 1H NMR (300 MHz, CDCl3, 25 °C): d = 0.50 (m, 2H, ASiCH2A), 1.15 (t, 9H, AOCH2CH3, J = 7.1 Hz), 1.97 (m, 2H, ACCH2CA), 3.72 (q, 6H, AOCH2CH3, J = 7.0 Hz), 4.30 (t, 2H, AN3CH2CH2A, J = 7.2 Hz), 5.22 (s, 2H, AOCH2A), 6.99 (d, 2H, H2, H6, J = 9.0 Hz), 7.54 (s, 1H, TzAH), 7.00–7.31 (m, 5H, H8AH12), 7.77 (d, 2H, H3, H5, J = 9.0 Hz), 8.28 (s, 1H, ACH@N). 13C NMR (75 MHz, CDCl3, 25 °C): d = 7.5 (SiCH2), 18.5 (CH3), 24.1 (CCH2C), 52.4 (N3CH2CH2), 58.5 (OCH2CH3), 62.7 (OCH2), 113.0 (C2, C6), 121.6 (C8, C12), 122.8 (C4), 125.9 (C10), 128.2 (C9, C11), 129.2 (C3, C5), 121.2, 132.8 (TzAC), 153.0 (C7), 156.0 (CH@N), 158.5 (C1). Synthesis of 2-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4yl)methoxy)benzylidene)pyridin-2-amine (3c). The quantities used were as: 2c (1.0 g, 4.24 mmol), AzPTES (1.05 g, 4.25 mmol). Yield: 1.81 g, 89%. Anal. Calcd. for C24H33N5O4Si: C, 59.60; H, 6.88; N, 14.48. Found: C, 59.53; H, 6.86; N, 14.53. IR (Neat, cm1): 754, 1013 (SiAO), 937 (CAC), 1161, 1259 (OACH2), 1352 (CH2AN), 1482 (CH3AC), 1682 (C@N), 2931 (C@CAH). 1H NMR (300 MHz, CDCl3, 25 °C): d = 0.49 (m, 2H, ASiCH2A), 1.12 (t, 9H, AOCH2CH3, J = 7.1 Hz), 1.94 (m, 2H, ACCH2CA), 3.70 (q, 6H, AOCH2CH3, J = 7.0 Hz), 4.27 (t, 2H, AN3CH2CH2A, J = 7.2 Hz), 5.29 (s, 2H, AOCH2A), 6.95–7.12 (m, 3H, H2AH4), 7.20–7.76 (m, 4H, H5, H8AH10), 7.58 (s, 1H, TzAH), 8.16 (d, 1H, H11, J = 9.0 Hz), 8.40 (s, 1H, ACH@N). 13C NMR (75 MHz, CDCl3, 25 °C): d = 7.4 (SiCH2), 18.3 (CH3), 24.1 (CCH2C), 52.4 (N3CH2CH2), 58.4 (OCH2CH3), 62.7

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(OCH2), 113.0 (C2), 113.1 (C8), 116.7 (C6), 121.4 (C4), 122.7 (C10), 128.3 (C5), 128.9 (C3), 133.3 (C9), 121.5, 135.9 (TzAC), 137.9 (C11), 158.8 (C7), 159.1 (CH@N), 160.6 (C1).

triethanolamine (0.27 g, 1.81 mmol), 3a (1.0 g, 2.07 mmol). M.p. 172–174 °C. Yield: 0.95 g, 93%. Anal. Calcd. for C25H31N5O4Si: C, 60.83; H, 6.33; N, 14.19. Found: C, 60.39; H, 5.96; N, 13.89. IR (Neat cm1): 592 (N ? Si), 753, 1100 (SiAO), 955 (CAC), 1160, 1242 (OACH2), 1368 (CH2AN), 1679 (C@N), 2982 (C@CAH). 1H NMR (300 MHz, CDCl3, 25 °C): d = 0.34 (m, 2H, ASiCH2A), 1.91 (m, 2H, ACCH2CA), 2.71 (t, 6H, ACH2NA, J = 6.0 Hz), 3.65 (t, 6H, AOCH2CH2A, J = 6.0 Hz), 4.25 (t, 2H, AN3CH2CH2A, J = 7.5 Hz), 5.22 (s, 2H, AOCH2CA), 6.96–7.19 (m, 3H, H2AH4), 7.28–7.39 (m, 5H, H8AH12), 7.56 (s, 1H, TzAH), 8.09 (d, 1H, H5, J = 9.0 Hz), 8.82 (s, 1H, ACH@N). 13C NMR (75 MHz, CDCl3, 25 °C): d = 13.0 (SiCH2), 26.2 (CCH2C), 50.9 (OCH2CH2N), 53.4 (N3CH2CH2), 57.4 (OCH2CH2), 62.6 (OCH2), 112.7 (C2), 121.2 (C6), 121.4 (C4), 125.1 (C8, C12), 125.7 (C10), 127.7 (C9, C11), 129.1 (C5), 132.8 (C3), 122.9, 143.1 (TzAC), 152.8 (C7), 156.6 (CH@N), 158.4 (C1). MS: m/z (relative abundance (%)): 94 (37), 132 (12), 150 (19), 174 (17), 297 (22), 419 (21), 441 (42), 457 (100). HRMS (ES+) Calcd. for C25H31N5O4Si: [M-74 + K]+ 457.1309, found 457.1303.

Synthesis of 4-((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4yl)methoxy)benzylidene)pyridin-2-amine (3d). The quantities used were as: 2d (1.0 g, 4.24 mmol), AzPTES (1.05 g, 4.25 mmol). Yield: 1.77 g, 87%. Anal. Calcd. for C24H33N5O4Si: C, 59.60; H, 6.88; N, 14.48. Found: C, 59.36; H, 6.75; N, 14.16. IR (Neat, cm1): 753, 1079 (SiAO), 958 (CAC), 1163, 1238 (OACH2), 1389 (CH2AN), 1484 (CH3AC), 1689 (C@N), 2974 (C@CAH). 1H NMR (400 MHz, CDCl3, 25 °C): d = 0.51 (m, 2H, ASiCH2A), 1.17 (t, 9H, AOCH2CH3, J = 7.1 Hz), 1.99 (m, 2H, ACCH2CA), 3.75 (q, 6H, AOCH2CH3, J = 7.0 Hz), 4.33 (t, 2H, AN3CH2CH2A, J = 7.2 Hz), 5.29 (s, 2H, AOCH2A), 6.98 (d, 2H, H2, H6, J = 8.0 Hz), 7.01–8.02 (m, 6H, H3, H5, H8AH11, ArAH), 7.54 (s, 1H, TzAH), 8.51 (s, 1H, ACH@N). 13 C NMR (75 MHz, CDCl3, 25 °C): d = 7.5 (SiCH2), 18.5 (CH3), 24.3 (CCH2C), 52.4 (N3CH2CH2), 58.5 (OCH2CH3), 62.8 (OCH2), 113.1 (C2, C6), 116.5 (C8), 122.6 (C10), 125.3 (C4), 128.8 (C3, C5), 135.7 (C9), 121.3, 143.1 (TzAC), 150.9 (C11), 157.5 (C7), 160.4 (CH@N), 161.9 (C1).

Synthesis of 4-((1-(3-(silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)benzylidene)benzenamine (4b). The quantities used were as: triethanolamine (0.27 g, 1.81 mmol), 3b (1.0 g, 2.07 mmol). M.p. 178–180 °C. Yield: 0.96 g, 94%. Anal. Calcd. for C25H31N5O4Si: C, 60.83; H, 6.33; N, 14.19. Found: C, 60.56; H, 6.17; N, 14.06. IR (Neat, cm1): 578 (N ? Si), 757, 1093 (SiAO), 940 (CAC), 1165, 1239 (OACH2), 1303 (CH2AN), 1630 (C@N), 2930 (C@CAH). 1H NMR (300 MHz, CDCl3, 25 °C): d = 0.32 (m, 2H, ASiCH2A), 1.91 (m, 2H, ACCH2CA), 2.72 (t, 6H, ACH2NA, J = 6.0 Hz), 3.66 (t, 6H, AOCH2CH2A, J = 6.0 Hz), 4.26 (t, 2H, AN3CH2CH2A, J = 7.5 Hz), 5.19 (s, 2H, AOCH2A), 7.00 (d, 2H, H2, H6, J = 9.0 Hz), 7.07–7.28 (m, 5H, H8AH12), 7.57 (s, 1H, TzAH), 7.76 (d, 2H, H3, H5, J = 9.0 Hz), 8.28 (s, 1H, ACH@N). 13C NMR (75 MHz, CDCl3, 25 °C): d = 13.0 (SiCH2), 26.3 (CCH2C), 51.2 (OCH2CH2N), 53.1 (N3CH2CH2), 57.7 (OCH2CH2), 62.4 (OCH2), 115.2 (C2, C6), 122.7 (C8, C12), 125.7 (C4), 129.2 (C10), 129.9 (C9, C11), 130.8 (C3, C5), 121.0, 140.3 (TzAC), 152.7 (C7), 159.5 (CH@N), 161.2 (C1). MS: m/z (relative

General procedure for the synthesis of silatranes To the stirred solution of triethanolamine (1 equiv) in toluene (30 ml) taken in a round bottomed flask fitted with a dean stark assembly, was added silane (1 equiv) drop wise followed by the addition of catalytic amount of KOH. The mixture was refluxed for 4 h in order to azeotropically remove ethanol formed during the reaction. Then the solvent was removed under reduced pressure followed by addition of 15 ml hexane. The contents were left for overnight stirring after which the product was isolated as dark brown solid which was filtered under nitrogen and dried under vacuum. Synthesis of 2-((1-(3-(silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)benzylidene)benzenamine (4a). The quantities used were as:

Ar

O

N

(F1)

O

-Ar-NH2

Si N

O N

O

+

N

N

O

+ O

Si

O

H2O

N

O N

O

[M-74+H]+

M = 493

N

N

(F2)

-N2

O

O

-

O

O

Si N

O

(F4)

O

NH H2C+

N

[M-235+K]+

. K+ (F5)

O

N

(F3)

-

Si O

N

O

O

O

Si

N

(255)

N H

O

. O

O

.

(R1)

O

Common Silatrane Fragmentation

Si O

N

(216)

(F6)

Scheme 2. General mass fragmentation of modified Schiff base silatranes (4a–4d).

G. Singh et al. / Journal of Molecular Structure 1079 (2015) 173–181

2a 4a

Fig. 1. UV spectrum of compounds 2a and 4a.

abundance (%)): 94 (11), 132 (5), 150 (20), 174 (12), 419 (100), 457 (45). HRMS (ES+) Calcd. for C25H31N5O4Si: [M-74 + H]+ 418.1672, found 418.1665.

Synthesis of 2-((1-(3-(silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)benzylidene)pyridin-2-amine (4c). The quantities used were as:

177

triethanolamine (0.27 g, 1.81 mmol), 3c (1.0 g, 2.07 mmol). M.p. 180–182 °C. Yield: 0.95 g, 93%. Anal. Calcd. for C24H30N6O4Si: C, 58.28; H, 6.11; N, 16.99. Found: C, 58.25; H, 6.16; N, 16.96. IR (Neat, cm1): 582 (N ? Si), 754, 1094 (SiAO), 937 (CAC), 1161, 1259 (OACH2), 1352 (CH2AN), 1682 (C@N), 2931 (C@CAH). 1H NMR (300 MHz, CDCl3, 25 °C): d = 0.34 (m, 2H, ASiCH2A), 1.90 (m, 2H, ACCH2CA), 2.72 (t, 6H, ACH2NA, J = 6.0 Hz), 3.66 (t, 6H, AOCH2CH2A, J = 6.0 Hz), 4.24 (t, 2H, AN3CH2CH2A, J = 7.5 Hz), 5.25 (s, 2H, AOCH2A), 6.96–7.13 (m, 3H, H2AH4), 7.20–7.75 (m, 4H, H5, H8AH10), 7.61 (s, 1H, TzAH), 8.14 (s, 1H, H11), 8.42 (s, 1H, ACH@N). 13C NMR (75 MHz, CDCl3, 25 °C): d = 13.1 (SiCH2), 26.2 (CCH2C), 51.0 (OCH2CH2N), 53.4 (N3CH2CH2), 57.5 (OCH2CH2), 62.7 (OCH2), 113.2 (C2), 117.3 (C8), 123.0 (C6), 125.3 (C4), 128.5 (C10), 128.6 (C5), 136.1 (C3), 137.9 (C9), 121.3, 142.8 (TzAC), 148.2 (C11), 156.5 (C7), 158.5 (CH@N), 160.9 (C1). MS: m/z (relative abundance (%)): 95 (72), 150 (3), 174 (28), 297 (40), 419 (32), 441 (100), 457 (65). HRMS (ES+) calcd. for C24H30N6O4Si: [M-74 + Na]+ 441.1569, found 441.1558. Synthesis of 4-((1-(3-(silatranyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)benzylidene)pyridin-2-amine (4d). The quantities used were as: triethanolamine (0.27 g, 1.81 mmol), 3d (1.0 g, 2.07 mmol). M.p. 181–183 °C. Yield: 0.94 g, 92%. Anal. Calcd. for C24H30N6O4Si: C, 58.28; H, 6.11; N, 16.99. Found: C, 58.23; H, 6.12; N, 16.96. IR (Neat, cm1): 583 (N ? Si), 756, 1097 (SiAO), 937 (CAC), 1123, 1276 (OACH2), 1351 (CH2AN), 1632 (C@N), 2921 (C@CAH). 1H NMR (400 MHz, CDCl3, 25 °C): d = 0.33 (m, 2H, ASiCH2A), 1.91 (m, 2H, ACCH2CA), 2.71 (t, 6H, ACH2NA, J = 6.0 Hz), 3.66 (t, 6H,

Fig. 2. Ellipsoid model showing molecular structure of compound 2b in the crystal.

Fig. 3. Ellipsoid model showing molecular structure of compound 4b in the crystal.

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G. Singh et al. / Journal of Molecular Structure 1079 (2015) 173–181

AOCH2CH2A, J = 6.0 Hz), 4.24 (t, 2H, AN3CH2CH2A, J = 7.5 Hz), 5.15 (s, 2H, AOCH2A), 6.88 (d, 2H, H2, H6, J = 8.0 Hz), 6.97–7.63 (m, 4H, H8AH11), 7.56 (s, 1H, TzAH), 7.83 (d, 2H, H3, H5, J = 8.0 Hz), 8.62 (s, 1H, ACH@N). 13C NMR (75 MHz, CDCl3, 25 °C): d = 13.2 (SiCH2), 26.4 (CCH2C), 51.0 (OCH2CH2N), 53.5 (N3CH2CH2), 57.5 (OCH2CH2), 62.3 (OCH2), 112.6 (C2, C6), 116.9 (C8), 122.9 (C4), 128.4 (C10), 131.8 (C3, C5), 137.9 (C9), 121.2, 143.2 (TzAC), 151.9 (C11), 157.5 (C7), 157.7 (CH@N). MS: m/z (relative abundance (%)): 95 (72), 150 (3), 174 (28), 297 (40), 419 (32), 441 (100), 457 (65), 495 (8). HRMS (ES+) calcd. for C24H30N6O4Si: [M-74 + Na]+ 441.1569, found 441.1556.

Results and discussion Synthesis Schiff bases were used as starting materials for the synthesis of MSBS (4a–4d) as shown in Scheme 1. Firstly, Schiff bases (1a–1d) were converted into terminal alkynes (2a–2d) by the reaction with propargyl bromide assisted by base K2CO3 in DMF (Supplementary Information). These Schiff base modified terminal alkynes were then coupled with AzPTES via CuAAC reaction to form 1,2,3-triazole blended triethoxysilanes (3a–3d). This methodology is based on highly efficient [CuBr(PPh3)3]-THF/Et3N system. Finally, transesterification of synthesized triethoxysilanes with triethanolamine in the presence of catalytic amount of KOH in toluene yielded MSBS (4a–4d) in good yield (Table 1). These silatranes were observed to be hydrolytically more stable than their corresponding triethoxysilane analogues. This idea is highly supported by the presence of hydrogen bonded water in the crystal structure of silatrane 4b. All the synthesized compounds were well characterized by various spectroscopic techniques [IR, NMR (1H, 13C), and Mass] and elemental analysis. The spectroscopic data of all the

compounds is given in supplementary material. Molecular structures of 2b and 4b are authenticated by single crystal X-ray crystallography. Spectroscopic analyses The IR spectra of newly synthesized compounds were recorded in the range 4000–400 cm1. The absorption frequencies were tuned to be in agreement with the structure of prepared compounds. All the compounds show strong absorption bands in the range 1598–1689 cm1, typical of CAH and C@N stretching vibrations of imine bond. The stretching frequency of C„C appears in the domain 2118–2124 cm1 for 2a–2d and disappearance of this peak in the compounds 3a–3d and 4a–4d confirms the cyclization of alkynyl moiety into triazole unit. In addition to this, symmetrical deformational vibrations of silatranyl skeleton are observed in the region 578–1491 cm1. Hypervalency in 4a–4d is approved by N ? Si stretching frequencies in region 578–592 cm1. Multinuclear (1H and 13C) NMR spectra are well consistent with the structure of synthesized compounds. In the 1H spectra of all compounds, a singlet in the region d = 8.28–8.84 ppm corresponds to imine proton. Alkynyl proton (for 2a–2d) appears around d  2.40–2.54 ppm. In 1H NMR spectra of cyclized compounds (for 3a–3d and 4a–4d), appearance of a peak due to triazole proton at d  7.48–7.61 ppm confirms the cyclization of alkynyl moiety into triazole unit. Additionally, shifting of AOCH2 protons of alkynyl group from d  4.68–4.80 ppm to d  5.15–5.29 ppm certifies the formation of cyclized product. Parallel shifting is observed in the carbon spectrum of the compounds. A triplet at d  1.12–1.17 ppm and quartet at d  3.70–3.75 ppm affirms the presence of triethoxysilyl group in compounds 3a–3d. In 4a–4d, two equivalent triplets appear around d  2.7 ppm and d  3.6 ppm due to ANCH2 and AOCH2 protons respectively of Si(OCH2CH2)3N skeleton.

Table 2 Crystal and structure refinement data of 2b and 4b. 2b

4b

Empirical formula Formula weight T (K) k (Å) Crystal system Space group

C16H13NO 235.27 100 0.71073 Monoclinic P21/c

C25H33N5O5Si 511.65 100 1.54178 Monoclinic P21/n

Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalcd (g cm3) Absorption coefficient (mm1) F (0 0 0) Crystal size (mm) Theta range for data collection (°)

12.0539(17) 7.9670(6) 12.9213(9) 90 95.722(6) 90 1234.7(2) 4 1.27 0.08 496 0.12  0.08  0.07 3.01–25.02

7.3113(3) 22.2180(9) 15.9851(6) 90 94.248(3) 90 2589.53(18) 4 1.31 1.18 1088 0.1  0.1  0.08 3.41–66.50

Indices range Reflections collected Independent reflections Completeness Absorption correction Max. and Min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) Largest diff. peak and hole

14 6 h 6 14, 9 6 k 6 9, 12 6 l 6 15 6035 2157 [R(int) = 0.0292] 99.2 Semi-empirical from equivalents 0.9945 and 0.9906 Full-matrix least-squares on F2 2157/0/163 1.03 R1 = 0.0365, wR2 = 0.0898 R1 = 0.0511, wR2 = 0.0974 0.184 and 0.179

8 6 h 6 7, 24 6 k 6 26, 17 6 l 6 19 17,470 4523 [R(int) = 0.0329] 98.8 Semi-empirical from equivalents 0.7528 and 0.6708 Full-matrix least-squares on F2 4523/91/371 1.03 R1 = 0.0467, wR2 = 0.0975 R1 = 0.0853, wR2 = 0.1147 0.207 and 0.222

G. Singh et al. / Journal of Molecular Structure 1079 (2015) 173–181

In 13C NMR spectra of all compounds the imine carbon appears as highly de-shielded carbon in the region d  156.0–160.4 ppm. Two carbons of alkynyl moiety (for 2a–2d) appear in the range of d = 76.1–78.6 ppm. In cyclized compounds (3a–3d and 4a–4d), appearance of triazole carbons around d  121.0–122.9 ppm and d  132.8–143.2 ppm confirms the cyclization of alkyne to triazole unit. In compounds 4a–4d, the peaks at d  50.9–51.2 ppm and d  57.4–57.7 ppm are assigned to ANCH2 and AOCH2 carbons of silatranyl moiety. Methylene carbon of propyl chain attached to silicon appears as the most shielded carbon at d  7.5 ppm (for 3a–3d) that on transesterification shifts to d  13.0 ppm (for 4a– 4d). This shift clearly indicates the hypervalency in the final compounds (4a–4d). Mass fragmentation pattern of MSBS 4a–4d (Scheme 2) shows interesting cleavage of Schiff base moiety followed by triazole fragmentation. Cleavage of imine bond results into fragments

Table 3 Hydrogen bonds of compounds 2b and 4b. D–H...A 2b C(3)AH(3B)...N(12)#1 C(18)AH(18)...O(4)#2 C(1)AH(1)...O(4)#3 4b O(1)AH(1A)O(2)#1 O(1)AH(1B)N(16)

d(D–H)

d(H...A)

d(D...A)

<(DHA)

0.99 0.95 0.95

2.62 2.52 2.36

3.607(2) 3.332(2) 3.219(2)

177.4 143.0 150.6

0.87 0.87

1.99 2.13

2.853(2) 2.956(3)

173.5 159.5

Symmetry transformations used to generate equivalent atoms: For 2b: #1 x, y + 1, z #2 x, y + 1/2, z + 1/2 #3 x + 2, y + 1/2, z + 1/2; for 4b: #1 x + 2, y + 1, z + 1.

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ArNH2 (F1) and C19H26N4O5Si (F2). The base peak appears at m/ z = 419 [M-74 + H]+ (for 4a), m/z = 457 [M-74 + K]+ (for 4b) or m/ z = 441 [M-74 + Na]+ (for 4c and 4d) corresponding to fragment F2 whereas a peak of low intensity appears at m/z = 494 corresponding to molecular ion [M + H]+. Fragment F1 appears as a peak at m/z = 94 [ArNH3]+ (for 4a, 4b) or m/z = 95 [ArNH3]+ (for 4c, 4d). This explains the stability of the atrane ring which does not get hydrolyzed at earlier stage of fragmentation. Further due to loss of nitrogen and fragment R1, a peak is observed at m/z = 255 (F4) or at its rearranged fragment (F5) m/z = 297 [M-235 + K]+. Further pattern follows the general silatrane fragmentation as discussed in literature [23]. Fig. 1 shows the UV–visible absorption spectra for 2a and 4a using methanol as solvent. Their absorption spectra are compared and a small red shift is observed for the final compound (4a) which shows absorbance at 282 nm as compared to the starting alkyne (2a) at 270 nm. The plausible explanation for this shift may be attributed to electron donating effect of triazole moiety in 4a. Similar shift is observed in the absorption spectra of other compounds.

Single crystal X-ray crystallography Colourless crystals of 2b and 4b suitable for single crystal X-ray crystallography were grown from the concentrated solution of respective compounds in chloroform. The molecular structures of 2b and 4b along with their atomic labelling are shown in Figs. 2 and 3. X-ray crystal data and structure refinement details of both compounds are listed in Table 2. X-ray diffraction study has shown that the compound 2b crystallized in the monoclinic crystal system (space group P21/c). In the crystal structure of 2b, the molecule

Fig. 4. Fragment of the crystal packing of 2b showing the CAH  O and CAH  N interactions involved in the stabilisation of the 3D network. Symmetry codes: $1  x + 2, y + 1/2, z + 1/2; $2x, y + 1, z; $x + 3, y + 1/2, z + 1/2.

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adopts a non-planar conformation, in which the N12 and the O4 atoms nearly lie in the plane of the corresponding phenyl ring and the dihedral angle between the aromatic rings is 42.67°. The most prominent feature of the crystal packing of 2b is the presence of C—H  O and C—H  N interactions (Table 3), involving the terminal acetylene group. The CAH (aromatic) and methylene groups act as donors meanwhile the methoxy-O and N12 (benzamine) atoms act as acceptors, which serve to link molecules in the supramolecular architecture (Fig. 4). The crystal packing is stabilized through weak C—H  p interactions. For 4b, X-ray diffraction study has shown that the compound crystallized in the monoclinic crystal system (space group P21/n). Selected bond lengths and bond angles are summarized in Table 4. The coordination polyhedron of the silicon atom in 4b is the common one for silatrane derivatives. The silicon surrounding is better described as distorted trigonal bipyramidal coordination, with the N1 and C11 as trans-apical donors and bond angle N1ASi1AC11 is found to be 179.12(10)°. The Si1AN1 bond is the largest apical coordination bond which is 2.138(2) Å and the three oxygen atoms are located in equatorial positions. All five-membered rings of the silatrane skeleton in 4b adopt an envelope-like conformation.

Table 4 Selected interatomic bonds lengths (Å) and trans angles (°) in compound 4b. Lengths (Å) Si(1)AO(8) Si(1)AO(2) Si(1)AO(5) Si(1)AN(1) Si(1)AC(11)

1.6575(18) 1.6793(17) 1.6589(18) 2.138(2) 1.869(3)

Angles (°) O(8)ASi(1)AO(2) O(8)ASi(1)AO(5) O(8)ASi(1)AN(1) O(8)ASi(1)AC(11) O(2)ASi(1)AN(1) O(2)ASi(1)AC(11) O(5)ASi(1)AO(2) O(5)ASi(1)AN(1) O(5)ASi(1)AC(11) C(11)ASi(1)AN(1)

120.44(9) 118.76(9) 83.65(9) 96.62(11) 83.07(8) 96.08(10) 116.97(9) 83.68(9) 96.91(11) 179.12(10)

The carbon atoms in the b-positions to the N atom occupy flap sites, while the C-a atoms are part of the base of the envelope plane. In the crystal, pairs of molecules form dimeric structures by hydrogen bonding interactions involving water molecules and the N16 atom from the triazole moiety and the O2(silatrane) atom as shown in Fig. 5 and Table 3. The packing of 4b revealed no significant p,p-stacking interactions. The bond angles around silicon atom may be discussed in terms of percentage trigonal bipyramidal character. The penta coordinate character can be calculated from three apical-to-equatorial bond angles and three equatorial-to-equatorial bond angles applying following equations:

X o i 109:5  1=3 hn =ð109:5  90:0Þ hn X  o i % TBPeq ¼ 100% : 1=3 /n  109:5 =ð120:0  109:5Þ % TBPax ¼ 100% :

hn

where hn is average of angles OeqASiACax and /n is average of angles OeqASiAOeq. % TBPax and % TBPeq for compound 4b are 66.46% and 87.87% respectively. Thermal analysis Thermal behavior of the compounds 2a and 4a was studied by TGA (Supplementary Information). The TGA of alkyne 2a reveals consistent mass loss. The compound decomposed in three steps. First mass loss corresponds to the elimination of AC„CH moiety. The second mass loss is equivalent to the loss of AC13H10N leaving behind AOCH2 moiety which further decomposes completely. TGA of compound 4a shows the mass loss due to formation of ethanol in the first step corresponding to loss of one side arm of silatranyl moiety. The residue left corresponds to the oxides of silica. Similar pattern is observed in the TGA of other compounds.

Fig. 5. Pair of molecules of 4b connected by H-bonding interactions involving water molecules and the N16 and O2 atoms. Symmetry code: $1  x + 2, y + 1, z + 1.

G. Singh et al. / Journal of Molecular Structure 1079 (2015) 173–181

Conclusions The present work is an ongoing research program aiming at the utilization of relatively simple and accessible Schiff bases to evolve novel silatranes. The synchronization of Schiff base, 1,2,3-triazole and silatranyl moiety in the final compounds is done in a skilful manner. The synthesized compounds can find myriad of applications in sol–gel processes, polymer chemistry, biological systems and various commercial products. Imine and triazole nitrogens can be exploited for the further coordination of these silatranes with various metal ions. Acknowledgments The authors are thankful to CSIR, New Delhi for providing financial support. Special thanks to Mr. Avtar Singh, SAIF, Panjab University, Chandigarh for NMR studies. The project ‘‘Factoría de Cristalización, CONSOLIDER INGENIO-2010’’ provided X-ray diffraction facilities for this work. Appendix A. Supplementary material CCDC 971354 and 971355 contain the supplementary crystallographic data for compounds 2b and 4b. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre. Copies of this information may be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: 44 1223 336 033; e-mail: [email protected] or www: http:// www.ccdc.cam.ac.uk).Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.molstruc.2014.09.042. References [1] H.C. Kolb, M.G. Finn, K.B. Sharpless, Angew. Chem. Int. Ed. 40 (2001) 2004– 2021. [2] K. Burglova, N. Moitra, J. Hodacova, X. Cattoen, M.W.C. Man, J. Org. Chem. 76 (2011) 7326–7333. [3] M. Murata, M. Ishikura, M. Nagata, S. Watanabe, Y. Masuda, Org. Lett. 4 (2002) 1843–1845. [4] K.N. Moitra, J.J.E. Moreau, X. Cattoen, M.W.C. Man, Chem. Commun. 46 (2010) 8416–8418. [5] G. Singh, S.S. Mangat, J. Singh, A. Arora, R.K. Sharma, Tetrahedron Lett. 55 (2014) 903–909. [6] V.V. Rostovtsev, L.G. Green, V.V. Fokin, K.B. Sharpless, Angew. Chem., Int. Ed. 41 (2002) 2596–2599. [7] M. Meldal, C.W. Tornoe, Chem. Rev. 108 (2008) 2952–3015. [8] D. Plazuk, B. Rychlik, A. Blauz, S. Domagala, J. Organomet. Chem. 715 (2012) 102–112. [9] J. Raushel, V.V. Fokin, Org. Lett. 12 (2010) 4952–4955. [10] F. Amblard, J.H. Cho, R.F. Schinazi, Chem. Rev. 109 (2009) 4207–4220. [11] Beena, N. Kumar, R.K. Rohilla, N. Roy, Rawat, Bioorg. Med. Chem. Lett. 19 (2009) 1396–1398.

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